If you've unlocked free crates they will remain on your account as the type of crate they are until opened.
They won't change type and shouldn't expire, just sit there as the type they are.
@Turelus - EU PC Megaserver "Don't count on others for help. In the end each of us is in this alone. The survivors are those who know how to look out for themselves."
Exactly that. I got Morrowind pretty recently. Unexpected the preorder bonus code was in it. I tried it, it worked, and got a dwarven crate, even if we were on reaper harvest already.